Plastic materials, such as polycarbonate (PC) and polymethylmethyacrylate (PMMA), are currently being used in the manufacturing of numerous automotive parts and components, such as B-pillars, headlamps, and sunroofs. Automotive rear window (backlight) systems represent an emerging application for these plastic materials due to many identified advantages in the areas of styling/design, weight savings, and safety/security. More specifically, plastic materials offer the automotive manufacturer the ability to reduce the complexity of the rear window assembly through the integration of functional components into the molded plastic system, as well as to distinguish their vehicle from a competitor's vehicle by increasing overall design and shape complexity. The use of a light weight rear lift gate module may facilitate both a lower center of gravity for the vehicle (better vehicle handling & safety) and improved fuel economy. Finally, enhanced safety is further recognized through a greater propensity for occupant or passenger retention with in a vehicle having plastic windows when involved in a roll-over accident.
Although there are many advantages associated with implementing plastic windows, these plastic modules are not without limitations that represent technical hurdles that must be addressed prior to wide-scale commercial utilization. Limitations, relating to material properties, include the stability of plastics to prolonged exposure to elevated temperatures and the limited ability of plastics to conduct heat. In order to be used as a rear window or backlight on a vehicle, the plastic material must be compatible with the use of a defroster or defogging system. In this respect, a plastic backlight must meet the performance criteria established for the defrosting or defogging of rear glass windows.
The difference in material properties between glass and plastics becomes quite apparent when considering heat conduction. The thermal conductivity of glass (Tc=22.39 cal/cm-sec-° C.) is approximately 4–5 times larger than that exhibited by a typical plastic (e.g., Tc for polycarbonate=4.78 cal/cm-sec-° C.). Thus a heater grid or defroster designed to work effectively on a glass window may not necessarily be efficient at defrosting or defogging a plastic window. The low thermal conductivity of the plastic may limit the dissipation of heat from the heater grid lines across the surface of the plastic window. Thus at a similar power output a heater grid on a glass window may defrost the entire viewing area of the window, while the same heater grid on a plastic window may only defrost the portion of the viewing area that is close to the heater grid lines.
A second difference between glass and plastics that must be overcome is related to the electrical conductivity exhibited by a printed heater grid. The thermal stability of glass as demonstrated by a relatively high softening temperature (e.g., Tsoften>>1000° C.) allows for the sintering of a metallic paste to yield a substantially inorganic frit or metallic wire on the surface of the glass window. The softening temperature of glass is significantly larger than the glass transition temperature exhibited by a plastic resin (e.g., polycarbonate Tg=145° C.). Thus for a plastic window, a metallic paste cannot be sintered, but rather must be cured at a temperature lower than the Tg of the plastic resin.
A metallic paste typically consists of metallic particles dispersed in a polymeric resin that will bond to the surface of the plastic to which it is applied. The curing of the metallic paste provides a conductive polymer matrix consisting of closely spaced metallic particles dispersed through out a dielectric polymer. The presence of a dielectric layer (e.g., polymer) between dispersed conductive particles leads to a reduction in the conductivity or an increase in resistance exhibited by cured heater grid lines as compared to dimensionally similar heater grid lines sintered onto a glass substrate. This difference in conductivity between a heater grid printed on glass and one printed on a plastic window manifests itself in poor defrosting characteristics exhibited by the plastic window as compared to the glass window.
Therefore, there is a need in the industry to design a heater grid that will effectively defrost and defog a plastic window in a manner similar to that performed on a glass window. Furthermore, there is a need in the industry to design a heater grid that will allow a printed metallic paste to perform as a defroster on a plastic window in a fashion similar to that exhibited by a printed heater grid on a glass window.